/* Reset and base styles */
* {
    margin: 0;
    padding: 0;
    box-sizing: border-box;
}

:root {
    --primary-color: #2563eb;
    --primary-hover: #1d4ed8;
    --secondary-color: #64748b;
    --accent-color: #f59e0b;
    --background: #ffffff;
    --surface: #f8fafc;
    --text-primary: #1e293b;
    --text-secondary: #64748b;
    --border: #e2e8f0;
    --shadow: 0 1px 3px 0 rgb(0 0 0 / 0.1);
    --shadow-lg: 0 10px 15px -3px rgb(0 0 0 / 0.1);
    --radius: 0.5rem;
}

body {
    font-family: 'Inter', -apple-system, BlinkMacSystemFont, 'Segoe UI', Roboto, sans-serif;
    line-height: 1.6;
    color: var(--text-primary);
    background-color: var(--background);
}

.container {
    max-width: 1200px;
    margin: 0 auto;
    padding: 0 1rem;
}

/* Header */
.header {
    background: linear-gradient(135deg, var(--primary-color), var(--primary-hover));
    color: white;
    padding: 3rem 0;
    text-align: center;
}

.logo {
    font-size: 2.5rem;
    font-weight: 700;
    margin-bottom: 0.5rem;
}

.subtitle {
    font-size: 1.125rem;
    opacity: 0.9;
}

/* Main content */
.main {
    padding: 3rem 0;
}

.intro {
    text-align: center;
    margin-bottom: 4rem;
}

.intro h2 {
    font-size: 2rem;
    font-weight: 600;
    margin-bottom: 1rem;
}

.intro p {
    font-size: 1.125rem;
    color: var(--text-secondary);
    max-width: 600px;
    margin: 0 auto;
}

/* Problems grid */
.problems-grid {
    margin-bottom: 4rem;
}

.problems-grid h2 {
    font-size: 1.875rem;
    font-weight: 600;
    margin-bottom: 2rem;
    text-align: center;
}

.problems-container {
    display: flex;
    flex-direction: column;
    gap: 3rem;
}

/* Category sections */
.category-section {
    margin-bottom: 2rem;
}

.category-title {
    font-size: 1.5rem;
    font-weight: 600;
    margin-bottom: 1.5rem;
    color: var(--text-primary);
    border-bottom: 2px solid var(--primary-color);
    padding-bottom: 0.5rem;
}

.category-problems {
    display: grid;
    grid-template-columns: repeat(auto-fit, minmax(300px, 1fr));
    gap: 1.5rem;
}

.problem-card {
    background: var(--surface);
    border: 1px solid var(--border);
    border-radius: var(--radius);
    padding: 1.5rem;
    transition: all 0.2s ease;
    text-decoration: none;
    color: inherit;
    display: block;
}

.problem-card:hover {
    transform: translateY(-2px);
    box-shadow: var(--shadow-lg);
    border-color: var(--primary-color);
}

.problem-card h3 {
    font-size: 1.25rem;
    font-weight: 600;
    margin-bottom: 0.5rem;
}

.problem-card .problem-number {
    display: inline-block;
    background: var(--primary-color);
    color: white;
    padding: 0.25rem 0.75rem;
    border-radius: var(--radius);
    font-size: 0.875rem;
    font-weight: 500;
    margin-bottom: 0.75rem;
}

.problem-card .problem-description {
    color: var(--text-secondary);
    font-size: 0.875rem;
    margin-bottom: 1rem;
}

.problem-card .problem-tags {
    display: flex;
    flex-wrap: wrap;
    gap: 0.5rem;
}

.problem-card .tag {
    background: var(--accent-color);
    color: white;
    padding: 0.25rem 0.5rem;
    border-radius: var(--radius);
    font-size: 0.75rem;
    font-weight: 500;
}

/* Features section */
.features {
    background: var(--surface);
    padding: 3rem 0;
    margin: 0 -1rem;
}

.features h2 {
    font-size: 1.875rem;
    font-weight: 600;
    margin-bottom: 2rem;
    text-align: center;
}

.features-grid {
    display: grid;
    grid-template-columns: repeat(auto-fit, minmax(250px, 1fr));
    gap: 2rem;
}

.feature-card {
    text-align: center;
    padding: 1.5rem;
}

.feature-icon {
    font-size: 3rem;
    margin-bottom: 1rem;
}

.feature-card h3 {
    font-size: 1.25rem;
    font-weight: 600;
    margin-bottom: 0.75rem;
}

.feature-card p {
    color: var(--text-secondary);
}

/* Footer */
.footer {
    background: var(--text-primary);
    color: white;
    padding: 2rem 0;
    text-align: center;
}

.footer a {
    color: var(--accent-color);
    text-decoration: none;
}

.footer a:hover {
    text-decoration: underline;
}

/* Responsive design */
@media (max-width: 768px) {
    .logo {
        font-size: 2rem;
    }
    
    .problems-container {
        grid-template-columns: 1fr;
    }
    
    .features-grid {
        grid-template-columns: 1fr;
    }
}

/* Loading animation */
.loading {
    text-align: center;
    padding: 2rem;
    color: var(--text-secondary);
    font-style: italic;
}

.loading::after {
    content: '';
    display: inline-block;
    width: 1rem;
    height: 1rem;
    border: 2px solid var(--border);
    border-radius: 50%;
    border-top-color: var(--primary-color);
    animation: spin 1s ease-in-out infinite;
    margin-left: 0.5rem;
}

@keyframes spin {
    to { transform: rotate(360deg); }
}

/* Error state */
.error {
    text-align: center;
    padding: 2rem;
    color: #dc2626;
    background: #fef2f2;
    border: 1px solid #fecaca;
    border-radius: var(--radius);
    margin: 2rem 0;
}